In the framework of certain applications, the antennas must have a wide band of operating frequencies, for example of the order of a decade, in other words a frequency band whose maximum frequency is equal to at least ten times the minimum frequency. Circular-polarized planar wire antennas, such as spiral antennas, belong to these wide frequency band antennas. A spiral antenna is generally composed of a dielectric substrate into which a radiating element is etched. The radiating element comprises at least two strands wound into a spiral whose inner ends are supplied with current. The electromagnetic radiation from the spiral antenna varies depending on the number of strands and the phase of the current in each strand. The width of the frequency band depends on the inner and outer diameters of the spiral.
From a theoretical point of view, a planar wire antenna possesses a plane of symmetry and therefore radiates into the whole of space, in particular in the two directions orthogonal to the plane of the antenna. For reasons of electromagnetic compatibility, the antennas must not interfere with the other systems situated nearby. Consequently, they are very often specified so as to radiate into a half-space. For this reason, the antenna is associated with a reflector which transforms the bidirectional radiation into a unidirectional radiation. From a practical point of view, this reflector also serves as a support allowing the antenna to be made more rigid and to be supplied with current.
According to a first solution, the reflector comprises an electrically conducting plane disposed at a distance from the antenna equal to a quarter of the mean wavelength of the radiation that it emits or that it receives. At such a distance, the electric field of the reflected backward radiation is then in phase with the electric field of the forward radiation. The main drawback of this solution is that the distance can only be adjusted in an optimal manner for a single wavelength. The electric field of the radiation emitted or received at wavelengths far from this mean wavelength therefore risk being affected, thus limiting the bandwidth of the antenna. Another drawback of this solution is that a quarter of a wavelength quickly corresponds to a large distance for low frequencies, which quickly leads to an overall relatively large thickness for the antenna. Furthermore, the electrically conducting plane allows the propagation of surface currents and reflection and scattering phenomena occur at the edge of the antenna, thus generating spurious radiation.
According to a second solution, the antenna reflector comprises a structure of the Artificial Magnetic Conductor (AMC) type disposed under the plane of the antenna on the side of the backward radiation. A conventional AMC structure comprises a dielectric substrate, electrically-conducting patterns disposed periodically on a first surface of the dielectric substrate and a uniform electrically-conducting plane forming a ground plane on a second surface of the dielectric substrate. Each conducting pattern can be connected to the ground plane via interconnection holes, generally referred to as “vias” in the literature. An AMC structure possesses the property of reflecting the electric field of the backward radiation in phase with the electric field of the forward radiation. It can therefore be positioned very close to the antenna and allows a reduction in the thickness of the antenna device comprising the antenna and the AMC structure. An AMC structure can also possess the property of prohibiting the propagation of electromagnetic waves in certain directions of the plane in which the conducting patterns are disposed, which prevents any spurious radiation from being generated. This is referred to as an electromagnetic band gap (EBG) structure. However, the properties of a structure of the EBG or AMC type are only manifest within a certain band of frequencies, referred to either as EBG band or as AMC band depending on the case in question. This band of frequencies, notably its central frequency and its low and high cutoff frequencies, depends on the shape and on the dimensions of the conducting patterns, and also on the thickness and on the relative permittivity of the dielectric substrate of the structure. In particular, for a relatively limited thickness of the dielectric substrate, in other words very small compared to the wavelength, whether either the EBG band or the AMC band are considered, the bandwidth is very narrow, in other words much less than an octave. Thus, the constraints relating to the thickness mean that the current antennas comprising a reflector with an EBG or AMC structure do not allow operation over a wide band of frequencies, greater than a decade.